Printed circuit board unit with detachment mechanism for electronic component

ABSTRACT

A printed circuit board unit comprises an insulated film disposed between a printed circuit board and an electronic component so as to define a through hole for receiving the solder bump. The through hole may be designed to form a constriction in the solder bump. Electric connection can reliably be established between the printed circuit board and the electronic component since the solder bump is allowed to penetrate through the through hole in the insulated film. When the insulated film is brought away from the printed circuit board, the insulated film serves to tear the solder bump in two pieces at the constriction, so that the electronic component can easily be detached from the printed circuit board.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a printed circuit board unit comprisinga printed circuit board and an electronic component such as a chipmounted on the surface of the printed circuit board through solders.

2. Description of the Prior Art

Soldering is often employed to mount electronic components such assemiconductor devices or bare chips on a printed circuit board in amulti-chip module (MCM) or a semiconductor package. Soldering is alsoemployed to mount electronic components such as the MCMs on a printedcircuit board of a larger size, namely, a motherboard. When theindividual bare chip or MCM is to be exchanged in the MCM ormotherboard, solders must be broken or removed to release bondingbetween input/output terminals on the bare chip or the MCM andinput/output pads arranged on the printed circuit board. For example, anexchanging operation is designed to detach the older bare chip or MCMfrom the printed circuit board when the solders are subjected to heat oftemperature higher than the melting point of the solders. A new barechip or MCM is thereafter mounted on the printed circuit board bysoldering input/output terminals of the new bare chip or MCM to theinput/output pads which have previously received the input/outputterminals of the old bare chip or MCM.

It is well known that part of the old solder still remains on theinput/output pad on the printed circuit board even after the old barechip or MCM is detached in the above-described exchanging operation.When a new bare chip or MCM is mounted on the printed circuit board, anew solder of a predetermined amount is added to the old solderremaining on the input/output pad. An excessive solder may induce ashort between the adjacent input/output terminals.

In particular, the respective input/output pads are not expected to keepsolders of a uniform amount remaining thereon. When a bare chip or MCMemploys a plurality of input/output terminals such as a ball grid array(BGA) and a pin grid array (PGA), the bare chip or MCM may suffer fromthe solders of a varied height on the input/output pads. Theinput/output terminals of the new bare chip or MCM may in part fail totouch the surface of the input/output pads on the printed circuit board.No electric connection can thus be achieved partly.

In view of the above inconvenience, the solders remaining on theinput/output pads should be wiped out before a new bare chip or MCM ismounted. For example, the printed circuit board along with the remainingbare chips or MCMs is subjected to another heat so as to melt thesolders remaining on the input/output pads. Heat is sometimes repeatedlyapplied to the printed circuit board more than twice until the soldersare completely wiped out. Repeated application of heat is supposed todamage and shorten the life of the printed circuit board, and bare chipsor MCMs remaining on the printed circuit board. It sometimesdeteriorates the wetness of the input/output pads to solders.

SUMMARY OF THE INVENTION

It is accordingly an object of the present invention to provide aprinted circuit board capable of avoiding repeated application of heateven when an electronic component is exchanged.

According to the present invention, there is provided a printed circuitboard unit comprising: a printed circuit board; an electronic component;a solder bump interposed between the printed circuit board and theelectronic component so as to fix the electronic component to theprinted circuit board; and an insulated film disposed between theprinted circuit board and the electronic component so as to define athrough hole for receiving the solder bump.

With the above structure, electric connection can reliably beestablished between the printed circuit board and the electroniccomponent since the solder bump is allowed to penetrate through thethrough hole in the insulated film disposed between the printed circuitboard and the electronic component. The insulated film is highlyexpected to contribute to detachment of the electronic component fromthe printed circuit board.

For example, the through hole may be designed to form a constriction inthe solder bump between the printed circuit board and the electroniccomponent. When the insulated film is brought away from the printedcircuit board, the insulated film serves to tear the solder bump in twopieces at the constriction, so that the electronic component can easilybe detached from the printed circuit board.

In this case, the insulated film is preferably superposed on the printedcircuit board so as to form the constriction right on a conductive padon the printed circuit board. Such constriction serves to allow thesolder bump of a constant amount to remain on the conductive pad, sothat a plurality of solder bumps of a uniform amount are allowed toremain on the corresponding conductive pads. The thinner the insulatedfilm gets, the less the solder bump remains on the conductive pad.

Such a through hole, standing on the conductive pad so as to form theconstriction in the solder bump, may have the inner peripheral sizesmaller than the outer peripheral size of the conductive pad. Such athrough hole may serve to prevent the melting solder bump from flowingoff the periphery of the surface of the conductive pad. In general, theconductive pad usually comprises a base conductive layer on a substrateof the printed circuit board, and a surface conductive layer superposedon the top surface of the base conductive layer. The surface conductivelayer is designed to have a corrosion resistance higher than the baseconductive layer. If the solder bump is prevented from flowing aroundthe surface conductive layer so as to reach the base conductive layer inthe above manner, it is possible to reliably avoid erosion of the baseconductive layer such as a copper layer. The base conductive layer isprevented from getting thinner or smaller. The surface conductive layermay be a nickel layer.

When the electronic component is to be detached from the printed circuitboard, the through hole may be displaced along the surface of theconductive pad. The solder bump should be kept at a melting temperature.The sliding movement of the insulated film along the surface of theprinted circuit board serves to completely wipe out the melting solderbump from the conductive pad. The solder bump is torn apart. Moreover,the solder bump hardly remains on the surface of the conductive pad.

Furthermore, the inner surface of the through hole may be covered with acoating wet to the solder bump. In detaching the electronic componentfrom the printed circuit board in this case, a relative movement may becaused between the conductive pad and the through hole while the solderbump is kept at a melting temperature. When the inner surface of thethrough hole moves across the solder bump, the melting solder bump isdragged by the coating having a higher wetness to the solder bump. Thesolder bump can be removed.

Furthermore, the thickness of the insulated film may be designed tocorrespond to the height of the solder bump on the printed circuitboard. Such an insulated film serves to surround the solder bump,received on the conductive pad, within the inner wall of the throughhole. In detaching the electronic component from the printed circuitboard, a relative movement may be caused between the conductive pad andthe through hole while the solder bump is kept at a melting temperature.The inner wall of the through hole is designed to completely wipe outthe solder bump from the conductive pad. Moreover, the solder bumphardly remains on the surface of the conductive pad.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other objects, features and advantages of the presentinvention will become apparent from the following description of thepreferred embodiments in conjunction with the accompanying drawings,wherein:

FIG. 1 is a plan view schematically illustrating a multi-chip module(MCM) according to a first embodiment of the present invention;

FIG. 2 is an enlarged partial sectional view of the MCM for illustratingthe structure of a solder bump;

FIG. 3 is an enlarged partial sectional view of the MCM for illustratinga method of mounting the MCU on the printed circuit board;

FIG. 4 is an enlarged partial sectional view of the MCM for illustratingthe method of mounting the MCU on the printed circuit board;

FIG. 5 is an enlarged sectional view illustrating the structure of aninput/output pad on the printed circuit board;

FIG. 6 is an enlarged partial sectional view of the MCM of the firstembodiment for illustrating a method of detaching the MCU from theprinted circuit board according to a specific example;

FIG. 7 is an enlarged partial sectional view of the MCM of the firstembodiment for illustrating a subsequent method of detaching the MCUfrom the printed circuit board according to the specific example;

FIG. 8 is an enlarged partial sectional view of the MCM of the firstembodiment for illustrating a method of detaching the MCU from theprinted circuit board according to another specific example;

FIG. 9 is an enlarged partial sectional view of the MCM of the firstembodiment for illustrating a subsequent method of detaching the MCUfrom the printed circuit board according to the another specificexample;

FIG. 10 is an enlarged sectional view schematically illustrating a partof an MCM according to a second embodiment of the present invention;

FIG. 11 is an enlarged partial sectional view of the MCM of the secondembodiment for illustrating a method of detaching the MCU from theprinted circuit board according to a specific example;

FIG. 12 is an enlarged partial sectional view of the MCM of the secondembodiment for illustrating a subsequent method of detaching the MCUfrom the printed circuit board according to the specific example;

FIG. 13 is an enlarged partial sectional view of the MCM of the secondembodiment for illustrating a method of detaching the MCU from theprinted circuit board according to another specific example;

FIG. 14 is an enlarged partial sectional view of the MCM of the secondembodiment for illustrating a subsequent method of detaching the MCUfrom the printed circuit board according to the another specificexample;

FIG. 15 is an enlarged partial sectional view of the MCM of the secondembodiment for illustrating a method of detaching the MCU from theprinted circuit board according to a further specific example;

FIG. 16 is an enlarged partial sectional view of the MCM of the secondembodiment for illustrating a subsequent method of detaching the MCUfrom the printed circuit board according to the further specificexample;

FIG. 17 is an enlarged sectional view of the MCM for schematicallyillustrating an insulated film as a seal member for the solder bumps;

FIG. 18 is an enlarged sectional view schematically illustrating a partof an MCM according to a third embodiment of the present invention;

FIG. 19 is an enlarged partial sectional view of the MCM of the thirdembodiment for illustrating a method of detaching the MCU from theprinted circuit board according to a specific example; and

FIG. 20 is an enlarged partial sectional view of the MCM of the thirdembodiment for illustrating a subsequent method of detaching the MCUfrom the printed circuit board according to the specific example.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 illustrates a multi-chip module (MCM) as a printed circuit boardunit according to a first embodiment of the present invention. The MCM10 comprises a printed circuit board 13, and a plurality ofsemiconductor devices or bare chips, such as a micro controller unit(MCU) 11 and central processing units (CPUs) 12, mounted on the printedcircuit board 13. A conductive pattern or printed circuit pattern, notshown, is formed on the printed circuit board 13 so as to electricallyconnect the MCU 11 and the CPUs 12, for example. The conductive patternmay spread over the surface of the printed circuit board 13, orinterposed between layers of insulator sequentially superposed to formthe substrate of the printed circuit board 13.

As is apparent from FIG. 2, a conductive pad or input/output pad 15 isformed on the surface of the printed circuit board 13 so as to receive asolder bump 14. The input/output pad 15 may be integral to theaforementioned conductive pattern over the surface of the printedcircuit board 13. Otherwise, the input/output pad 15 may be connected tothe aforementioned conductive pattern by a conductive via, for example.The solder bump 14 is designed to receive an input/output terminal orterminal pad 16 arranged on the rear surface of the MCU 11. The solderbump 14 serves to fix or bond the MCU 11 on the printed circuit board13.

An insulated film 17 is disposed between the surface of the printedcircuit board 13 and the MCU 11. The insulated film 17 may be made frompolyimide, for example. The thickness of the insulated film 17 is set ina range of 5-10 μm while the solder bump 14 has a height ofapproximately 70 μm. Such height can be measured in the verticaldirection upright to the surface of the printed circuit board 13. Theinsulated film 17 is superposed on the surface of the input/output pad15. A through hole 18 is defined or provided in the insulated film 17 soas to receive the solder bump 14. The through hole 18 is designed todefine or form a constriction 14 a in the solder bump 14 between the MCU11 and the printed circuit board 13 since the inner peripheral size orinner diameter of the through hole 18 is set smaller than the outerperipheral size or outer diameter of the solder bump 14.

When the MCU 11 is mounted on the printed circuit board 13 in the MCM10, the insulated film 17 is first superposed on the surface of theprinted circuit board 13, as shown in FIG. 3. The input/output pads 15are exposed at the surface of the printed circuit board 13. The throughholes 18 have previously been formed in the insulated film 17. Anexcimer laser may be employed to bore the insulated film 17, forexample. The position of the through holes 18 is set to correspond toarrangement of the input/output pads 15. When the insulated film 17 issuperposed on the printed circuit board 13, the through hole 18 isdesigned to define a column space standing upright on the surface of theinput/output pad 15, as is apparent from FIG. 3.

The MCU 11 is thereafter set on the surface of the insulated film 17.The MCU 11 has previously been provided with solder bumps 14 on therespective terminal pads 16. When the terminal pads 16 on the rearsurface of the MCU 11 is aligned with the corresponding input/outputpads 15 on the printed circuit board 13, the solder bumps 14 are adaptedto close the upper openings of the corresponding through holes 18,respectively, as shown in FIG. 4. After the MCU 11 has been set on theprinted circuit board 13 in this manner, the printed circuit board 13 isintroduced in a furnace for a heat treatment. The solder bumps 14 meltunder the atmosphere of a melting temperature to thereby flow into thecorresponding through holes 18. When the printed circuit board 13 istaken out of the furnace and cooled, the solder bumps 14 get hardened onthe surface of the input/output pads 15, as shown in FIG. 2.

The input/output pad 15 comprises, for example, a circular copper layer21 on the surface of the synthetic resin or ceramic substrate, acircular nickel layer 22 superposed on the top surface of the copperlayer 21, and a gold layer 23 superposed on the top surface of thenickel layer 22, as shown in FIG. 5. If a solder containing tin contactsthe copper layer 21, a higher temperature of the heat treatment mayinduce absorption of the copper layer 21 into the solder. The nickellayer 22 serves prevent such erosion of the copper layer 21. If theinner peripheral size or inner diameter of the through hole 18 is setsmaller than the outer peripheral size or outer diameter of the surfaceof the input/output pad 15 in the above-described manner, the throughhole 18 serves to reliably prevent the melting solder bump 14 fromreaching the copper layer 21 by flowing around the nickel layer 22. Thecopper layer 21 can reliably be prevented from erosion and gettingthinner or smaller.

When any defect of the MCU 11 is found in the MCM 10, the MCU 11 ispreferably replaced with a new MCU 11. When the version of an installedsoftware should be updated, the MCU 11 of the old version may bereplaced with an MCU 11 of the new version. In any event, such exchangeof the individual MCUs 11 may contribute to reuse of the printed circuitboard 13 and the remaining electronic components such as the CPUs 12mounted on the printed circuit board 13.

When the MCU 11 is to be exchanged, the old MCU 11 is first detachedfrom the printed circuit board 13. In detachment, a heat block 24 isfirst allowed to contact the MCU 11, as shown in FIG. 6, for example.The MCU 11 is supposed to transmit heat from the heat block 24 to thesolder bump 14. The solder bump 14 is caused to melt. The MCU 11 is thenlifted above the printed circuit board 13, so that the solder bump 14 istorn between the input/output pad 15 and the terminal pad 16. Thebonding can be released in this manner between the MCU 11 and theprinted circuit board 13.

When the MCU 11 has been lifted, the solder bump 14 sometimes cancompletely be removed from the input/output pad 15 on the printedcircuit board 13. Part of the solder bump 14 may sometimes remain on theinput/output pad 15. When the insulated film 17 is then lifted andbrought away from the printed circuit board 13, as shown in FIG. 7, thesolder bump 14 is further torn in two pieces at the constriction 14 a.The constriction 14 a is only allowed to remain on the input/output pad15. The solder bumps 14 of a uniform amount are expected to remain onthe respective input/output pads 15.

After the solder bumps 14 are removed in the aforementioned manner, theinput/output pads 15 are allowed to receive solder bumps 14 of a new MCU11. Since the old solder bumps 14 of a uniform amount remain, all of thesolder bumps 14 of the new MCU 11 are expected to contact thecorresponding input/output pads 15. All signal paths corresponding tothe respective solder bumps 14 are reliably established between theconductive pattern on the printed circuit board 13 and the MCU 11.

It should be noted that the MCU 11 may be detached from the printedcircuit board 13 at the same time when the insulated film 17 is detachedfrom the input/output pads 15. In addition, the insulated film 17 may bedetached from the input/output pads 15, while the solder bumps 14 arekept melting, or after the solder bumps 14 get hardened.

Alternatively, when the MCU 11 is detached from the printed circuitboard 13 in the MCM 10, as shown in FIG. 8, the through hole 18 may bedisplaced along the surface of the input/output pad 15 while the solderbump 14 is kept at a melting temperature, for example. The slidingmovement of the insulated film 17 along the surface of the printedcircuit board 13, keeping contact of the heat block 24 with the MCU 11,serves to completely wipe out the melting solder bump 14 from theinput/output pad 15. When the MCU 11 is thereafter lifted up, as shownin FIG. 9, the MCU 11 holding the solder bump 14 can be detached fromthe printed circuit board 13. The solder bump 14 hardly remains on theinput/output pad 15.

FIG. 10 illustrates a part of an MCM 31 according to a second embodimentof the present invention. As is apparent from FIG. 10, the thickness dof the insulated film 17 is set to correspond to the height of thesolder bump 14 in this embodiment. In addition, the solder bump 14 issurrounded by the inner surface or wall of the through hole 18 since theinner peripheral size or inner diameter of the through hole 18 is setlarger than the outer peripheral size or outer diameter of the solderbump 14. It should be noted that the like reference numerals areattached to structure or components achieving the function or advantagesidentical to those in the above-described first embodiment.

When the MCU 11 is to be detached from the printed circuit board 13 inthe MCM 31, the inner wall of the through hole 18 may be displaced alongthe surface of the terminal pad 16 while the solder bump 14 is kept at amelting temperature, for example. As shown in FIG. 11, the slidingmovement of the MCU 11 along the stationary insulated film 17, keepingcontact of the heat block 24 with the MCU 11, serves to hinder theterminal pad 16 on the MCU 11 from dragging the melting solder bump 14.As a result, the solder bump 14 is torn off from the terminal pad 16 ofthe MCU 11, as shown in FIG. 12. After the MCU 11 has been detached fromthe printed circuit board 13, the solder bump 14 hardly remains on theterminal pad 16 on the MCU 11. Accordingly, the MCU 11 can easily bereused without additional operation for removing the solder bump 14 fromthe terminal pad 16. In particular, such method is most useful when anelectronic component is to be detached in a semiconductor package. Sucha semiconductor package in general employs a single bare chip, moreexpensive than the printed circuit board 13 or the remaining electroniccomponents on the printed circuit board 13.

On the other hand, the printed circuit board 13 may be slid along theinsulated film 17, for example, so as to achieve the displacement of thethrough hole 18 relative to the surface of the input/output pad 15, asshown in FIG. 13, while the heat block 24 is maintained on thestationary MCU 11. In this case, the inner wall of the through hole 18serves to hinder the input/output pad 15 on the printed circuit board 13from dragging the melting solder bump 14. As a result, the solder bump14 is torn off from the input/output pad 15 of the printed circuit board13, as shown in FIG. 14. After the MCU 11 has been detached from theprinted circuit board 13, the solder bump 14 hardly remains on theinput/output pad 15 on the printed circuit board 13. Accordingly, theprinted circuit board 13 can easily be reused without additionaloperation for removing the solder bump 14 from the input/output pad 15.In addition, if the insulated film 17 is slid simultaneously relative tothe MCU 11 and the printed circuit board 13, as shown in FIGS. 15 and16, the solder bump 14 can be wiped out from the terminal pad 16 on theMCU 11 and the input/output pad 15 on the printed circuit board 13,respectively. It should be noted that the insulated film 17 employed inthe above-described MCM 31 may be utilized as a sealing member for thesolder bumps 14, as shown in FIG. 17.

FIG. 18 illustrates a part of an MCM 41 according to a third embodimentof the present invention. In this embodiment, the inner surface of thethrough hole 18 is covered with a coating 42 which having a higherwetness to the solder bump 14. Note that the like reference numerals areattached to structure or components achieving the function or advantagesidentical to those in the above-described first and second embodiments.

When the MCU 11 is to be detached from the printed circuit board 13 inthe MCM 41, the inner wall of the through hole 18 may be displaced alongthe surface of the input/output pad 15 while the solder bump 14 is keptat a melting temperature, for example. In this case, the insulated film17 may, not only contact the terminal pad 16 on the MCU 11 and theinput/output pad 15 on the printed circuit board 13, but also keepspaced from the terminal and the input/output pad 16, 15. As shown inFIG. 19, when the insulated film 17 is moved relative to the stationaryMCU 11 and printed circuit board 13 in the horizontal direction, forexample, keeping contact of the heat block 24 with the MCU 11, thecoating 42 serves to drag the melting solder bump 14. The solder bump 14is torn off simultaneously from the terminal pad 16 of the MCU 11 andthe input/output pad 15 on the printed circuit board 13. Moreover, sincethe solder bump 14 is expected to held on the insulated film 17, asshown in FIG. 20, the solder bump 14 can easily be collected.

It should be noted that the present invention can be applied not only tothe above-described MCMs 10, 31, 41 but also to any type of printedcircuit board unit such as a motherboard comprising MCMs and otherelectronic components mounted on a printed circuit board. Heat air maybe employed to allow solder bumps to melt between the MCMs and otherelectronic components and the printed circuit board of a larger size,for example, as is conventionally known. Moreover, the present inventionmay be applied not only to a ball grid array (BGA) such as theaforementioned MCMs 10, 31, 41 but also to any type of solder bondingsuch as a pin grid array (PGA), a lead frame, and the like.

1-12. (canceled).
 13. A method of detaching an electronic component froma printed circuit board, comprising: keeping a solder bump at a meltingtemperature on a surface of a conductive pad on the printed circuitboard under the electronic component; and displacing a wall defined inan insulated film between the printed circuit board and the electroniccomponent so as to surround the solder bumps.
 14. The method ofdetaching according to claim 13, wherein the insulated film is displacedrelative to the conductive pad.
 15. The method of detaching according toclaim 13, wherein the insulated film is displaced relative to theelectronic component.